Revert "Revert "First step towards having patch files for generic libraries.""

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index 5719f01839f8d2243808af307787dabd3ab14bf1..0a81f15e6eeb98a6fae2dd910a7dd06cb4e3ea8f 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -156,6 +156,9 @@ class MockCompiler extends Compiler {
     // Do nothing. The mock core library is already handled in the constructor.
   }
 
+  // The mock library doesn't need any patches.
+  Uri resolvePatchUri(String dartLibraryName) => null;
+
   Script readScript(Uri uri, [ScriptTag node]) {
     String code = sources[uri.toString()];
     if (code === null) throw new IllegalArgumentException(uri);